Researchers have developed a new titanium-based material that is a good candidate for making lead-free, inorganic perovskite solar cells.

In a new paper, which appears in the journal Joule, the researchers show that the material is especially good for making tandem solar cells—arrangements in which a perovskite cells are placed on top of silicon or another established material to boost the overall efficiency.

Perovskites have emerged as a promising alternative to silicon for making inexpensive and efficient solar cells. But for all their promise, perovskites are not without their downsides. Most contain lead, which is highly toxic, and include organic materials that are not particularly stable when exposed to the environment.

“Titanium is an abundant, robust, and biocompatible element that, until now, has been largely overlooked in perovskite research,” says senior author Nitin Padture, professor of engineering and director of the Institute for Molecular and Nanoscale Innovation.

“We showed that it’s possible to use titanium-based material to make thin-film perovskites and that the material has favorable properties for solar applications which can be tuned.”

Perovskites’ popularity

Interest in perovskites, a class of materials with a particular crystalline structure, for clean energy emerged in 2009, when researchers discovered that they could convert sunlight into electricity.

The first perovskite solar cells had a conversion efficiency of only about 4 percent, but that has quickly skyrocketed to near 23 percent, which rivals traditional silicon cells. And perovskites offer some intriguing advantages. They’re potentially cheaper to make than silicon cells, and they can be partially transparent, enabling new technologies like windows that generate electricity.

“One of the big thrusts in perovskite research is to get away from lead-based materials and find new materials that are non-toxic and more stable,” Padture says.

“Using computer simulations, our theoretician collaborators at UNL (University of Nebraska-Lincoln) predicted that a class of perovskites with cesium, titanium, and a halogen component (bromine or/and iodine) was a good candidate. The next step was to actually make a solar cell using that material and test its properties, and that’s what we’ve done here.”

The team made semi-transparent perovskite films that had bandgap—a measure of the energy level of photons the material can absorb—of 1.8 electron volts, which is considered to be ideal for tandem solar applications. The material had a conversion efficiency of 3.3 percent, which is well below that of lead-based cells, but a good start for an all-new material, the researchers say.

“There’s a lot of engineering you can do to improve efficiency,” says coauthor Yuanyuan Zhou, an assistant professor (research) of engineering at Brown. “We think this material has a lot of room to improve.”

Min Chen, a PhD student of materials science and the paper’s first author, used a high-temperature evaporation method to prepare the films, but says the team is investigating alternative methods. “We are also looking for new low-temperature and solvent-based methods to reduce the potential cost of cell fabrication.”

Advantages over the alternatives

The research showed the material has several advantages over other lead-free perovskite alternatives.

One contender for a lead-free perovskite is a material made largely from tin, which rusts easily when exposed to the environment. Titanium, on the hand, is rust-resistant. The titanium-perovskite also has an open-circuit voltage—a measure of the total voltage available from a solar cell—of over one volt. Other lead-free perovskites generally produce voltage smaller than 0.6 volts.

“Open-circuit voltage is a key property that we can use to evaluate the potential of a solar cell material,” Padture says. “So, having such a high value at the outset is very promising.”

The researchers say that material’s relatively large bandgap compared to silicon makes it a prime candidate to serve as the top layer in a tandem solar cell. The titanium-perovskite upper layer would absorb the higher-energy photons from the sun that the lower silicon layer can’t absorb because of its smaller bandgap. Meanwhile, lower energy photons would pass through the semi-transparent upper layer to be absorbed by the silicon, thereby increasing the cell’s total absorption capacity.

“Tandem cells are the low-hanging fruit when it comes to perovskites,” Padture says. “We’re not looking to replace existing silicon technology just yet, but instead we’re looking to boost it. So if you can make a lead-free tandem cell that’s stable, then that’s a winner. This new material looks like a good candidate.”